Reply to ASK-AN-EARTH-SCIENTIST

By changing the shape of the seafloor. If an
earthquake ruptures all the way to (or even just close to)
the seafloor, then part of the seafloor will be raised or
lowered. Imagine that the seafloor is raised. Seawater
itself is almost incompressible, so by raising the
seafloor the earthquake will also be raising the sea
surface. The sea surface, however, likes to be horizontal.
An extra lump of water at the surface will collapse under
gravity to produce a series of waves, the tsunami. This is
directly analogous to dropping a pebble into a pond - there
you create a hole in the water surface; the hole in the
surface is gravitationally unstable, so it collapses to
give you a series of ripples. What makes tsunamis so
alien from our everyday experience is the fact that
they have exceptionally long wavelength (the distance
from one crest to the next). Because the area affected
by the earthquake may be huge, the wavelength of the
waves may be huge too - as much as 200 miles. What
that means is that individual wave crests may be as
much as half-an-hour apart. In other words, when a
tsunami hits a coastline, the edge of the water is
dangerous for hours.
Eleven people at Crescent City, California
died in 1964 and 59 people at Hilo, Hawaii, died in
1960 because they did not understand: the people
evacuated, but returned to shoreline houses or businesses
before it was safe. The fist waves of those tsunamis were
small; the later waves killed people.